1. Field of the Invention:
The present invention relates to a hydrostatic continuously variable transmission for transmitting drive power from a power source, such as an engine, to drive wheels of a working vehicle, for example.
2. Description of the Relevant Art:
Some working vehicles such as riding-type lawn mowers, tractors, etc. employ a hydrostatic continuously variable transmission comprising a hydraulic pump having a plurality of pistons and a movable swash plate, the hydraulic pump being actuatable by an engine, and a hydraulic motor having a plurality of pistons and a movable swash plate, the hydraulic motor being rotatable by a hydraulic pressure generated by the hydraulic pump. When the hydraulic motor is rotated, its rotating power is transmitted at a variable speed reduction ratio to the drive wheels of a working vehicle or the like.
If the swash plate of the hydraulic pump were not accurately held in a neutral position when it should be kept in the neutral position, the hydraulic pressure generated by the hydraulic pump would tend to leak to the hydraulic motor. Therefore, when the engine is in operation, the hydraulic motor would apply rotating power to the drive wheels, causing the vehicle to move. Alternatively, when the engine is started, the hydraulic pump would produce an abrupt pressure leakage causing the hydraulic motor to move the vehicle suddenly.
One solution to the above problem is disclosed in Japanese laid-open patent publication No. 62-253967. According to the disclosed axial-piston machine, an orifice is disposed in a bypass passage interconnecting high- and low-pressure fluid passages which are connected to a hydraulic pump and a hydraulic motor. When an unwanted hydraulic pressure is generated by the hydraulic pump due to the swash plate being shifted out of the neutral position, the hydraulic pressure is relieved by the orifice in bypassing relationship to the hydraulic motor, thereby preventing drive wheels or a vehicle from moving. The orifice is defined in a bypass valve that is manually movable into the bypass passage. The high-pressure fluid passage is normally held in communication with the bypass passage through the orifice when the bypass valve is seated on a valve seat in the bypass passage.
However, since the bypass valve is manually actuated by the operator, it may not be fully seated on the valve seat due to the operator's oversight. When the bypass valve is not seated during operation, oil flows at a high rate from the high-pressure fluid passage to the low-pressure fluid passage through the bypass passage, causing the hydraulic motor to stop or lower its rotational speed. Another problem is that since oil leaks from the high-pressure fluid passage through the orifice at all times, the volumetric efficiency and hence operation efficiency of the transmission are lowered while it is in operation.
U.S. Pat. No. 4,765,228 discloses an axial piston machine with a circuit flushing arrangement. The axial piston machine has a pressure valve in the form of a conical valve body disposed in a guide pin and preloaded by a compression spring.